Fixed NDData dep, however found a bug in astropy.

This commit relies upon astropy PR #1650
Fix MapMeta to allow it to be deepcopied.

This is now working after the merge of astropy #1686
bump astropy dep to 0.3.0 final
add extra numpy extractions to GenericMap
Merge pull request #634 from Cadair/issue_607

Make Map inherit from NDData